Process for making inoculation inserts

ABSTRACT

A new process is described in which an inoculation insert is made of a mix of inoculant of ferroalloys, additive of cryolite and binder of polyvinyl butyral. All the materials state in form of dry and loose powder at room temperature. When heat is provided the binder will change thermally cured and set and make the mix agglomerated by chemical bonding. The insert proves excellent compression strength and surface hardness that can ease special handling care during transportation and storage. The insert may be an individual article or impregnated inside the open cells of ceramic filters as an assembly by virtues of a great deal of flexibility provided by the process to fit unique requirement of application. The consistency can be greatly increased with the insert located in the runner system of the mold that the molten metal, particularly iron, can be inoculated.

TECHNICAL FIELD

The present invention is related to an improved process for making chemical bonded inoculation insert which affords more consistency in the inoculation of iron being cast. The inventive process provides an improved technique for putting selective amount of the inoculant on selective area of the filter combining the advantage of both inoculation and filtration for the manufacture of metal casting parts for which it is desired to obtain a structure free of defect.

BACKGROUND OF THE INVENTION

Standard metal casting metallurgical practice includes inoculation wherein the nucleation and growth of graphite is encouraged at the expense of iron carbide formation. Preferential nucleation greatly enhances the mechanical and physical properties of the finished casting. In the production of most cast iron the molten metal is treated with addition of various types of inoculants to provide the nucleation sites. Examples of suitable inoculants are graphite, calcium silicide and ferrosilicon, usually containing 50-85% by weight silicon and small quantities of calcium and/or aluminum. Special types of ferrosilicon containing other elements such as titanium, chromium, zirconium, manganese, copper, bismuth, alkaline earths such as barium or strontium, or rare earths such as cerium, may also be used. The size of the particles of inoculant may be up to about 10 millimeters but preferably particles having a narrow size range of less than 6 mm, more preferably 0.05-2.0 millimeters are used. Relatively large particles tend to produce slower fading of the inoculation effect because they dissolve in the molten iron relatively slowly but they may produce insufficient nucleation sites. Relatively small particles produce sufficient nucleation sites but because they dissolve faster they tend to produce more rapid fading.

Usually the beneficial effects of the inoculant addition are short-lived; therefore, it is best to make such additions as late in the casting cycle as possible. Attempts have been made to utilize an inoculant placed in the runner system of a mold, which is referred to as the in-mold inoculation process or add the inoculant along with the stream of the molten metal entering the mold, which is referred to as the in-stream inoculation process.

The form in fine particles is a simple and cheap one utilized in the in-mold and in-stream inoculation processes. But they have not been successful because the particles of inoculant tend to get blown into the mold cavity where they can form inclusions in the casting produced when the molten iron solidifies. There is a tendency for castings having variations in their microstructure to be produced because the method does not supply a consistent and accurate weight of additive and involve high maintenance costs for the additive placement equipment as well.

In order to overcome the problems associated with the use of fine particle methods have been proposed which utilize inserts made of bonded, compressed, sintered or cast particulate inoculants, over which the molten iron flows. However, none of these methods has been wholly successful and none has achieved wide commercial use because they tend to dissolve inconsistently or shatter under the influence of thermal shock that they can give rise to inclusions in the castings.

For instance, the binders used to make the bonded inoculants include wax, sodium silicate, rubber materials etc. The inserts made with the wax have very weak compression strength and surface that therefore require extra effort and incremental cost in handling during transportation and storage. They are known to dissolve very rapidly. The rapid dissolution rate has caused the inoculants to be overlooked in the interest due to the understanding that the rapid rate of dissolution would cause the insert to be dissolved prior to the end of the pour and therefore the inoculant would not be effective throughout the entirety of the pour. Also the considerable amount of gaseous decomposition from wax can give rise to porosity in the casting. All the issues have made the wax bonded insert difficult to control. The inserts made with sodium silicate or rubber materials have a bit better strength and surface than those made with wax but dissolve slow early in the pour. Most remarkably, all these bonded inserts have great tendency to shatter under the impact of thermal shock that they would cause inconsistent dissolution and inclusions in the casting. A cycle for making an insert with either of the above binders is significant and therefore obstacle to achieve the goals of increasing manufacturing productivity and decreasing cost.

The compressed inserts have no use of any binder but agglomerated by mechanic bonding when the fine particles in cavity of metal tooling is applied a great compressing pressure and the surface contacts between particles locks all particles and prevent from moving. The inserts have higher density than the chemically bonded ones and follow to some extent the slow dissolution rate pattern of the sintered and cast inserts to be discussed below.

The sintered and cast inserts have very good strength and surface but are known to dissolve very slowly due to the understanding that the inserts need significantly more heat energy and time than the binders to dissolve and therefore the rate of inoculation may be slow early in the pour and inconsistent and unpredictable throughout the entity of the pour. Besides, the high cost involved with manufacturing the inserts restricts a wide adoption of the approach to inoculating the molten metal for the economical concern.

For many years people have attempted to incorporate an inoculant with ceramic filter combining the advantages of the inoculation and filtration together. The filter having many open cells through the both opposite faces prevents large impurities like undissolved inoculant, oxidized inoculant and alloy slag from entering the mold cavity and assures the manufacture of metal casting parts free of the impurities. The British Patents GB 1105028 and 1257168 disclose the inoculant coated ceramic filter. The particles of inoculant may be dispersed in a liquid binder of wax and applied onto the filter by dipping or spraying etc. Similarly the Foseco Patent U.S. Pat. No. 5,033,531 describes a filter comprising the filter having their walls at least partially coated with a first layer of wax and a second layer of an inoculant. Due to the volatile fluidity by the nature of the liquid wax the weight of the inoculant coated on the filter is difficultly controlled. The automation control and maintenance associated with the equipment is very high.

The Daussan patent FR 2,692,654 describes a filter inoculant assembly wherein the insert is obtained by agglomeration of powder under a great compressing pressure. The efficiency of this filter inoculant assembly is quite limited due to the feature of the slow dissolution rate of the compressed insert.

The Foseco Patent EP 0 234 825 describes a filter inoculant assembly wherein the inoculant is presented in the form of a powdery non-agglomerated powder enclosed in a plastic pouch. This unit is more complex to manufacture and employs non-agglomerated powder whose wet ability with respect to the liquid cast iron is not always well controlled.

The U.S. Pat. No. 6,793,703 discloses a filter assembly comprising a filter and an inoculating pellet in contact with the filter. The pellet has a range of an inoculant dissolution rate. The pellet in contact with the filter occupies the opening area of passageway of the filter and therefore blocks the molten metal from flowing through the filter and therefore decreases the flow rate of the molten metal that would give rise to defects like cold shut, shrinkage, porosity due to the understanding that the molten metal turns cool and stop flowing when solidification takes place.

There has been a long-standing desire in the metal casting industry for an inoculating insert, and method of use, which insures consistent and predictable inoculation regardless of the amount and rate of the molten metal is poured. The method must provide technique that an insert may be made in precise amount and competitive cost. Prior to the present invention this desire has not been met.

OBJECTIVES OF THE INVENTION

It is an objective of the present invention to provide a new chemically bonded inoculation insert which rapidly and consistently inoculates the molten metal free of inclusions over a wide working range of metal amount and pour times without fading and ineffective inoculation.

It is another objective of the invention to provide a new and easy alternative process to localize the inoculant into the open cells of the filter to accomplish the goals of inoculating late in the metal casting process.

DESCRIPTION OF THE INVENTION

In this invention preparation of an inoculation insert follows. Prepare a metal tooling with cavity as the shape of the inoculation insert as required. Get the powdery materials of the present invention for the inoculation insert, which include the inoculant, additive and binder in size less than 2 millimeters each. Put the materials together at ratio and mix till a homogeneous mix is observed. Owning to the powdery state of the materials at room temperature the mix has excellent flow ability and allows mixing done during a short period of time, i.e., 2-3 minutes dependent on the quantity and mixer efficiency being employed. Load the mix into the cavity of metal tooling or the selected open cells of the filter by gravity or inject the mix into the cavity of the metal tooling or the selected open cells of the filter by compressed air. Leave the mix loaded metal tooling or filter in a furnace previously set at a temperature and for a certain period of time till the binder changes thermally cured and set across the insert. Actual employment of temperature and time may be dependent on specific size and shape of the inoculation insert being made. Remove the mix loaded metal tooling or filter from the furnace to cool down. Take the agglomerated mix out of the cavity of the metal tooling or keep the agglomerated mix loaded filter as assembly, both treated as a finished inoculation insert product ready for use. The entire process can be done without requiring sophisticated equipment for handling liquid material like wax or huge hydraulic compression pressure.

The cost associated with the equipment is kept in reasonable low level as compared to those for other types of the inserts.

The body forming the ceramic filter may be for example a ceramic body having a honeycomb type of structure having cells extending between the both opposite faces of the body. The pick-up of said mix of the present invention (hereinafter referred as “the inoculant”) by the filter will be dependent on the size and number of the open cells filled with the inoculant. For example for a square extruded ceramic filter 55 millimeter long, 55 millimeter wide and 12.5 millimeter thick having 2 open cells per linear filled every other cell with the inoculant the pick up is 20 grams of the inoculant using the inoculant of particle size 0.2 mm-0.5 mm. For the same sized filter with 4 smaller open cells per linear centimeter the pick up may be reduced to 12 grams because the smaller the size of the cells the less the volume of the cells available to contain the inoculant.

The unique characteristics of the binder of the present invention prove an ideal choice for the in-mold inoculation process. It has density 1.107 cm³/g and states in powder of while color at room temperature. Moisture absorption is below 4 percent that gives long self-life for storage requirement. It turns softening at relatively low temperature of 140-149° F. (60-65° C.) that polymerization reaction initiates and develops chemical bonding. The low temperature is a big advantage of the minimum usage of heating energy and time that they may be interpreted productivity and low cost to make the inoculant inserts. The compression strength of the inserts made with the binder is five times those made with wax. However, gas evolution out of the binder decomposition is only one fifth of wax and close to the low limit of rubber materials given the same compression strength. In the event the gaseous decomposition is of considerable volume it can be caught in the molten metal and form porosity defect in the metal casting parts, which has been a problem with the inserts made with wax. In the event the gaseous decomposition is of low volume it can be beneficial. The small amount of gaseous decomposition creates a thin separation layer against abrasion and thermal shock of the molten metal on the insert and therefore helps achieve consistent distribution of the substitute of the inoculant insert into the melt and prevent from shattering that would give rise to inclusions in the casting. The chemical stability of the binder is not high and allows the inoculant insert to dissolve rapidly into the molten metal. The dissolution rate of the insert made with the binder is between those made with wax and the rubber materials in the family of chemically bonded inserts and certainly faster than compressed, sintered and cast inserts.

Temperature of molten metal is a big factor affecting the dissolution rate of the insert. In practice the metal casting production has typical temperature variation by 122-212° F. (50-100° C.) during one pour that they would cause the inconsistent distribution of the substitute of the inoculant insert. To evaluate dependence of the binder decomposition on temperature of molten metal temperature sensitivity index (TSI) is designed by measuring amount of the dissolved substitute of the inoculant into the melt given a certain period of time. Experiment data prove the TSI number of the binder of the present invention is one tenth of rubber and one twenty fifth of the wax and therefore the dissolution rate of the inoculation insert of the present invention is least sensitive to the pour temperature deviation and therefore assures a fast but consistent distribution of the substitutes of the inoculant insert into the melt free of inclusions.

When the molten metal contacts the inoculation insert the additive of cryolite starts decomposing into solid phase of sodium fluoride (NaF) and gaseous phase of aluminum fluoride (AIF₃). It can reduce surface tension of the molten metal and increase the wet ability of the substitute of the inoculant insert with the molten metal and helps absorption of the substitute of the inoculant into the melt and consequently bring out the maximum efficiency of inoculation. In the meantime the gaseous phase AIF₃ serves as a separation layer against abrasion and thermal shock of the molten metal and consequently helps consistent distribution of the substitute of the inoculant into the melt.

For example, the inoculation insert is placed in the mold so that all of the molten metal has the opportunity to be exposed to the inoculation insert before entering the mold cavity, which shapes the final casting. When the first molten metal reaches the insert the heat from the molten metal pyrolizes the insert driving off the gaseous components generated from the binder and cryolite of the insert which then pass into the surrounding sand or through the mold vents. The substitute of the inoculant will be gradually dissolved as the char partially surrounding them is dissolved or oxidized away by reactions with the oxide particles trapped by the insert. This retardation of the dissolution of the inoculant insert proportions their effects among the entire batch of the molten metal entering the mold, minimizing the degree of mixing required distributing the inoculant uniformly within the casting. The advantages of this can easily be recognized. The inoculant substitute held in place improves the metallurgy of the casting by minimizing the fading of the beneficial effects of the inoculant during the time between reaction and solidification of the casting. The level of the binder and cryolite control the rate of reaction of the inoculant insert with the molten metal, distributing the benefits throughout the mass of metal entering the mold cavity.

In the application that the inoculant impregnated filter is placed in the mold the molten metal receives the substitute of the inoculant from the inoculant impregnated cells of the filter when it contacts the filter while the molten metal flows through the remaining open cells of the filter. Those open cells remain the same open surface during the pour and keep a constant flow rate of the molten metal during the pour. The outer surface of the inoculant in the filter exposed to the molten metal regulates incoming heat source from the molten metal that is a big factor affecting the dissolution rate of the substitute of the inoculant into the melt. Such the outer surface is a sum of the open surface of the cells filled with the inoculant and remains unchanged during the entire pour and affords a consistent dissolution rate of the inoculant and therefore a consistent distribution of the substitute of the inoculant till the inoculation in the filter is totally dissolved and the blocked cells reopen to allow the melt through in the late pour. While inoculating the molten metal the filter keeps the impurities away from entering the mold for the manufacture of metal casting free of inclusions. The quantity of the inoculation dissolved into the molten metal depends on the pick up of the inoculant previously put in the filter.

The inoculating insert of the invention offers the following advantages:

1) It has good compression strength and surface that can ease the special handling care during transportation and storage.

2) It permits due to characteristics of the binder and additive a fast and consistent distribution of the substitute of an inoculant in a metal stream and a reduction in the amount of inoculant required for effective treatment and defects like inclusion.

3) It enables any shape of the inoculation insert made upon job requirement.

4) It allows selection of the area and amount to be put in ceramic filter.

5) It provides a simple, easy and economical way to make the inoculation insert in a large economical scale.

6) It enables the use of a single method of applying both a filter and an inoculant in a mold cavity.

7) Incorporation of an inoculant with a filter reduces casting inclusions caused by undissolved inoculant, oxidized inoculant or alloy slag.

8) The insert is adaptable to automatic placement in a mold thus reducing manpower requirements.

The following examples will serve to illustrate the invention:

EXAMPLE 1

An inoculant insert weighing 90 grams was prepared with the process of the present invention comprising 75% silicon and balance iron. Hydraulic transmission housing castings were made in that cast iron comprising flake graphite and composed of 3.27% C, 1.07% Mn, 0.83% Cu, 1.91% Si, 0.046% S, 0.078% P and 0.46% Cr was poured at 2543° F. (1395° C.) into a horizontal divided mold. The melt was inoculated in the mold by means of an inoculation insert located on the base of down sprue of the pour. For demonstration purpose FIG. 2 provides a schematic of the runner system employed and location of the insert in accompany of a real sample by picture. The inoculant body was virtually disposed in the restrained melt and was able to dissolve approximately in proportion to the quantity of iron flowing through.

The time required to fill the mold amounted to 12-13 seconds. The comparison target is the late inoculation by in-stream process. The mechanical properties are demonstrated in Table 1. The metallurgical result of the inoculation was tested on a lot of 30 castings. For that purpose, samples were taken from the several portions and polished sections were made there from for an inspection of the structure. The polished section was also etched for a detection of cementite.

It is apparent from the table that tensile strength increases and length of graphite length decreases, even better than the results made by the in-stream inoculation process owning to the distinct efficiency of the inoculation treatment made with the in-mold inoculation insert.

EXAMPLE 2

A filter 55 millimeters long, 55 millimeters wide and 12.5 millimeters thick was filled by gravity every other cell with the inoculant comprising 75% silicon and balance iron. The incremental weight of the inoculant in the filter is 20 grams. The inoculant impregnated filter was then heated to 482° F. (250° C.) for two (2) minutes to let the binder thermally cured and set. After cooling the filter was examined under the microscope; it was determined that the particles of the ferroalloys were completely agglomerated by the binder.

FIG. 2 demonstrates an illustration of the filter in accompany of a real sample by picture. It was placed into the runner system of a green sand mold for the manufacture of diesel engine bed plates. The molten metal entering the mold was exposed to the outer surface of the inoculation insert, mixed together with the substitute of the inoculant while entering through the open cells of the filter. Approximately 100 pounds of gray cast iron for making auto engine part was poured through the filter in 12 seconds at a temperature of 2553° F. (1400° C.). Examination of the filters cells revealed that all of the inoculant had dissolved into the molten metal.

Appendix: TABLE 1 Mechanical properties of the hydraulic casting by the in-mold inoculation treatment. Microstructure Mechanical Property Pearlite Cell Strength Hardness Shrinkage Dimension Casting Factor Graphite Content % No. Mpa HB Tendency Accuracy Cleanness In Mold 2-6 mm, Type A >98 N/A 230 203 None +0.9-1.35 mm Good 0.1% L-55 mm In-Mold 5-10 mm, Type A >98 N/A 220 199 None +0.9-1.35 mm Good 0.1% L-54 mm In Stream Type A >98 N/A 217 195 None +0.9-1.35 mm Good 0.1% L-60 mm Pour 1530 out of furnace Temp C. 

1. A process for an inoculation insert made of a mix of inoculant of ferroalloys, additive of cryolite and binder of polyvinyl butyral that state in form of dry and loose powder at room temperature and are agglomerated by chemical bonding, comprising steps of: Weighing each of the powdery materials; Putting together the materials at ratio; Mixing mechanically all the materials at room temperature; Loading mechanically the mix into cavity of element; Leaving the mix loaded element in furnace at a temperature and for a period of time to allow the binder thermally cured and set; Removing the mix loaded element out of the furnace to cool down to the room temperature; Taking the agglomerated mix out of the element or keeping the mix loaded element ready for use.
 2. A mix as recited in claim 1 wherein said binder is selected from a group comprising essentiality of polyvinyl butyral and substances having similar physical and chemical characteristics of polyvinyl butyral, such as polyvinyl formal, polyvinyl acetaldehyde and polyvinyl acetate.
 3. A mix as recited in claim 1 wherein said additive is selected from a group comprising cryolite and substances having physical and chemical characteristics of cryolite.
 4. A mix as recited in claim 1 wherein said inoculant is selected from a group comprising essentiality of ferroalloys, such as ferrosilicon and other specialty alloys having size distribution in range of 0.05-2.0 millimeters for each.
 5. An inoculant insert as recited in claim 1 wherein said mix has ratio essentiality comprising by weight a range from 0 to 6 percent for said additive and a range from 3 to 10 percent for said binder and the balance for said inoculant.
 6. A temperature as recited in claim 1 wherein said furnace is heat elevated in a range from 392 to 662° F. (200-350° C.).
 7. A time as cited in claim 1 wherein said furnace is held at said temperature for a period in range from 10 seconds to 10 minutes.
 8. An inoculant insert as recited in claim 1 wherein said element is selected from a container group comprising metal body (tooling) having cavity and ceramic filter having cavity alike open cells through the both opposite faces.
 9. A process as recited in claim 1 said insert takes a number of the forms in use, comprising an individual article or assembly with ceramic filters.
 10. A process for molten cast iron in a mold having a cavity and a runner system utilizing said insert, comprising the steps of: locating said insert in the runner system of the mold; casting molten cast iron into a mold over outer surface of said insert in a runner system of the mould whereby said insert is dissolved in said melt. 